Toluene sweetening



Patented Aug.. 29, 1944 UNITED Gysbert F. de Ridder, Houston, Tex.,assignor to Shell Development Company, San Francisco, Calif., acorporation` of Delaware Application september 2'1, 1943, serial No.503,285

(cl. 19e-29 11 Claims.

This invention is concerned with the sweetening of relatively lowboilingaromatic hydrocarbons recovered from petroleum distillates withthe aid of selective solvents, particularly from naphthas.

For many years the recovery of relatively pure low boiling aromatichydrocarbons, specically benzene, toluene, xylenes, ethyl benzene, etc.,from petroleum oil has been considered impracticable on a commercialscale, and only in recent years methods have been developed which havemade such recovery feasible. Most of these methods employ selectivesolvents one way or another. Depending upon circumstances, liquid-liquidsolvent extraction may be employed, or vapor-liquid extraction (betterknown as extractive distillation), or azeotropical distillation.

Many diiferent types of solvents maybe employed in the separation. Forexample, liquid SO'.- is suitable for liquid-liquid extraction, as wellas most of the selective solvents which have in the past vbeen employedfor extracting kerosene or lubricating oils, as for example phenol,cresols, nitrobenzene, nitrotoluene, benzaldehyde, aniline, furfural,beta beta dichlor diethyl ether, crotonaldehyde, various sulfolanes assulfolane, methyl sulfolane, dimethyl sulfolane, methyl sulfolanylether, etc., and many others. Solventsv suitable for extractivedistillation have been described in Dunn, U. S. 2,288,126. Inazeotropical distillation, one may employ methyl alcohol, ethyl alcohol,acetone, methyl ethyl ketone, acetonitrile, propionitrile, nitromethane,nitroethane. SO2,

ammonia, methylamine, ethylamine, propylamine,

etc.

The petroleum naphthas which yield aromatic hydrocarbons may be straightrun or cracked or re-formed (both catalytically and non-catalytically),iso-formed, dehydrogenated, isomerized and dehydrogenated, etc.,distillates. Depending on the procedure employed, naphtha prior to theseparation of the aromatics may or may not contain mercaptans. Thepresent invention is concerned with the treatment of aromatichydrocarbons recovered by extraction from naphthas which do not containmercaptans, more particularly aromatic hydrocarbons having between 6 and10 carbon atoms per molecule and boiling below about 400 F., such asbenzene, toluene, xylenes, ethylbenzen'e, hemimellitene, pseudocumene,mesitylene, etc. l

In the separation of aromatic hydrocarbons from some naphthas with theaid of a selective covered by these methods, while otherwise of highpurity, contain in many instances relatively large amounts ofmercaptans. There are many methods known for refining hydrocarbon oilsfor the purpose of removing mercaptans, outstanding among them being theold sulfuric acid treatment and re-distillation. Sulfuric acidapparently may act in a dual capacity of an oxident and selectivesolvent, dissolving some mercaptans and converting others to relativelyhigh boiling disulfides, which may then be separated by fractionaldistillation, provided the naphtha has a relatively narrow boilingrange. Its drawback, however, is that it may fail to sweetencompletelywhere mercaptan concentration is relatively high. Whenattempting to sweeten sour hydrocarbons such as benzene, toluene, etc.,it was found that, as a rule, ordinary sulfuric acid treatment andre-distillation, Whether carried out in batch or in continuous treatingequipment, would not result in complete sweetening. In all these casesit was possible, however, to sweeten successfully when blowing duringacid treatment with a large amount of air, the only difficulty being inthis case that the amount of air required was so large that seriousvaporization losses of the aromatic hydrocarbon solvent, mercaptansaccumulate in the aromatic extract fraction.

occurred. In an eiort to overcome losses by vaporization, mechanicalagitation alone was tried in an attempt to Whip air into the mixture.This was also found to be inadequate in that not enough air could bewhipped into the hydrocarbon to completely oxidize the undesirableingredients and to produce a suiliciently sweet product unlessexcessively long agitation was employed, which made the processimpractical.

The purpose of this invention is to economically, effectively andsubstantially completely sweeten sour aromatic light hydrocarbondistillates recovered by solvent extraction of naphthas. Another purposeis to reduce the harmful mercaptarl content in such aromatichydrocarbons. A specic purpose is to sweeten sour 1 toluene recovered byextraction of petroleum distillates to produce nitration grade toluene.Other purposes will appear in the following description.

Now it has been discovered that sweetening of sour, relatively lowboiling aromatic hydrocarbons can be achieved quite easily and withoutsubstantial loss of the valuable hydrocarbon by sulfuric acid treatmentand re-distillation, if the sulfuric acid is thoroughly distributedthroughout the hydrocarbon liquid by mechanical means and simultaneouslya small controlled amount of airis sus-,- pended in the liquid togetherwith the acid. This amount of air is substantially less than thestoichiometrical amount required to convert all of the mercaptans in theoil to disuldes, 4and preferably is between about t4; and 1/2 of saidstoichiometrical amount. If less than this amount of air is employed thehydrocarbons will not be suiiiciently sweetened, and if a greater amountof air is employed unnecessary hydrocarbon losses will occur. lThe airtogether with the sulfuric acid serves to oxidize the last trace ofmercaptans not extracted by the sulfuric acid. In addition to reducinglosses by vaporization this procedure has the advantage of alwaysproducing a sweet product, and in cases where sweetening with acid alonewas possible, of requiring only about half as much acid as is necessaryto sweeten in the absence of air. The intensity of mixing theabovementioned limited amount of air, with the acid, and hydrocarbonsmust be at least such that during the entire contact time, which usuallylasts from about 1/3 up tov 10 hours, the air and acid remain suspendedin the hydrocarbon liquid. y In continuous treating equipment, contacttimes below about 8 hours are usually sufficient, while in batchtreatment, owing to the greater difllculty of thoroughly distributingthe air through a large volume of the hydrocarbon liquid, longer contacttimes are necessary as a rule. Y

Generally the steps in carrying out the process of this inventioncomprise: mechanically agitating the hydrocarbon with acid and air;separating the air from the acid and hydrocarbon; and, separating theacid from the hydrocarbon. In order to remove products of oxidation,sulfonation, etc., it is generally necessary to include the additionalsteps of:v neutralizing the hydrocarbon; and, re-distilling theneutralized product.

The accompanying drawing shoes a preferred flow diagram of the processof this invention.

Referring to the drawing, the sour aromatic hydrocarbons are introducedthrough valved line I into the mixer 2, wherein they are contacted withacid introduced through valved line 3. The resulting mixture is thenpassed through line 4, into a turbo-mixer or agitator 5, provided withan efcient stirrer 6, such as a lightning mixer, driven by a suitablemeans such as a. motor 1. A controlled amount of air is introduced intothe agitator through valved line 8, joining line 4. Steam may also beintroduced together with this air through valved line 9 to help to keepthe temperature of the mixture above about 75 F., and preferably above85 F. and below about 160- F., as well as toprevent clogging of theopenings in line 4. Optimum treating temperature range is between about110 F. and 120 F. Clogging may be caused by the formation of vandsulfuric acid react to form toluene-sulfonic acid which readilycrystallizes out of solution. At higher temperatures, however, theyremain in solution.

In the agitator the air, acid, and sour aromatic hydrocarbons areintensively mixed s0 that a suspension is produced of sufficientstability to prevent air separation in the agitator and to causeretention in the suspension of a major portion of the air while thelatter remains in the agitator. The residence time in the agitator isbetween about 1A; to 3 hours. As a result substantially all themercaptans in the hydrocarbons will either be extracted or oxidized.

This suspension of hydrocarbon, acid, and air passes from the top of theagitator 5, through line I0, into air separator II, provided-with an airvent line I2, containing automatic valve I I, operated by levers I5connected. to a liquid level float I4 in the chamber of the separatorII.

This automatically controlled valve I3 permits uniform and constantseparation of the air bubbles from the foam produced in the agitator iand minimizes the loss of hydrocarbon vapors. It has been founddesirable to separate the air from the suspension produced in theagitator 5 prior to the separation of the acid in that after the air isremoved the acid may be separated by mere settling.

The air-free mixture of acid and hydrocarbon liquid is withdrawn fromthe bottom of separator II, through line I6, into the acid separator I1,which preferably comprises a settler and coalescer, not shown. In thesettler the acid and hydrocarbon mixture is allowed to form two liquidphases, namely an upper hydrocarbon phase and a lower acid-sludge phase,the latter being Withdrawn vthrough valved line I8. phase is then passedthrough the above-mentioned coalescer such as a chamber iilled withpebbles or the like to remove amounts of acid which did not settle intothe acid-sludge phase. After this the hydrocarbon phase usually sti11contains a trace of acid which must be removed by neutralization. Forthis purpose it is passed through line I9, into mixer 20, wherein it ismixed with an aqueous alkaline solution such as aqueous sodiumhydroxide, sodium carbonate, etc., introduced through valved line 2l inan amount sufficient to neutralize residual acid. The resulting mixtureis then passed through line 22 into settler 23.

The neutralized oil is passed through line 25 to the still 28, providedwith heating means 21, for redistillation of the sweetened hydrocarbonThe redistilled and final pure and sweetened hydrocarbon product iswithdrawn from the top of still 26 through line 28, and the remainingsmall percent of high boiling impurities is withdrawn through the bottomvalved line 29 and usually discarded.

This redistillation of the aromatic hydrocarbons is preferably carriedout at a temperature not much above the boiling temperature of thearomatic product, in that excess heat tends to decompose the disuldesformed in the process, thereby souring it again. To avoid this it isalso desirable that copper be omitted in the bottom of the still 26, inthat copper acts as a catalyst causing decomposition of thesedisuliides. This latter catalytic eect, howeverI may be overcome byaddition of small amounts of ammonia and air during the distillation. Anordinary steel still has been found satisfactory in the redistillationof sweetened aromatic extracts. The amount of loss of aromatichydrocarbon by 'redistillation generally ranges between about 1% and21/% of the amount redistilled.

The sour aromatic hydrocarbons comprising the feed to this process mustbe free from the solvent used in extracting them from their respectivehydrocarbon distillates. For example, in the extractive distillation ofa toluene concentrate in accordance with Dunn et al. U. S. 2,288,126, inthe presence of phenolic solvent. the resulting toluene shouldpreferably be caustic alkali treated to remove all traces of the phenolprior to the sweetening treatment of this invention.

Generally, the amount of mercaptan sulfur present in such aromaticsobtained by extraction The upper as described is not more than about .5%by weight and usually is about .3% by weight or less depending upon thetype of crude from which it Was derived and its prior treatment,although in some cases it may exceed 1%.

The acid employed in treating the aromatic hydrocarbon is preferablyrelatively concentrated sulfuric acid having a concentration betweenabout 87% and 100%, preferably about 93% to 98%. This acid may be freshor reconcentrated and refortied spent acid which has primarily beenemployed in the same or other treating processes. The quantity of acidrequired is dependent primarily upon the composition of the souraromatic feed and the content of mercaptans and olens, as well as on theconcentration of the acid employed. Treating temperatures are anotherfactor, optimum temperatures being bev tween 110 F. and 120 F. Thus forexample in treating toluene, the acid requirement at 110 F. is onlyabout half that at 85 F. The quantity of acid required may also dependto a minor extent upon the mechanical design of the acid injectionapparatus or mixer 2. The quantity of acid usually ranges from about 2to 50 pounds per barrel of sour aromatic hydrocarbon, depending upon theconcentration of the acid, less acid being required the higher itsconcentration. Roughly, in the range of 87% to 100% concentration, 1%difference in the concentratloncorresponds to about 2 to 3 pounds perbarrel difference in the amount required. Thus, when using 98% to 100%sulfuric acid, amounts required normally vary from about 2 to 15 poundsper barrel; when using 93% to "98% acid the amounts vary between about 5and 25 pounds per barrel; and when using 87% to 89% acid the requirementis usually on the order of 30 to 50 pounds per barrel. More acid may beemployed if color tests, such as the acid heat test, of the resultingsweetened product do not meet specifications.

The following specific example clearly illustrates the processI of thisinvention:

Example I A toluene concentrate derived by distillation of a West Texaspetroleum naphtha was extractively distilled with phenol to produce asour, 1 toluene containing .3% by weight of mercaptan sulfur.Theoretically, one barrel of this sour toluene would require 13.5 cubicfeet of air to oxidize all of the mercaptan present in the toluene.pounds of 98%l sulfuric acid and 2 to 3 cubic feet of air per barrel oftoluene in a mixer, agitator and air separator exactly as shown in thedrawing. The residence time of the mixture of toluene, acid and air inthe agitator was about 35 minutes, and the treating temperature wasabout 85 F.

After removal 4of the air in the air separator, the acid was removed bysettling and coalescing. The resulting toluenewas then neutralized with2.25 to 2.75 pounds per barrel of 80 B. aqueous sodium khydroxidesolution andv redistilled in a still having a reboiler temperature ofnot over 300 F., whereby a doctor sweet toluene was produced meetingnitration grade specifications.

The loss of toluene caused by the sweetening treatment amounted tobetween 1% and 1.5%.

I claim as my invention: v

l. In a process for sweetening sour aromatic hydrocarbons of 6 to 10carbon atoms per molecule containing mercaptans, the steps comprisingcontacting them at a temperature between 75 and 160 F. with betweenabout 2 and 50 pounds per barrel of sulfuric'acid of 87100|%concentration and a controlled amount of air, mechanical- 'ly agitatingsaid mixture to produce a suspension of said acid and air in saidhydrocarbons, maintaining said suspension for a period of about 1/3 to10 hours and then separating said air and said acid from saidsuspension, said amount of air being substantially less than thestoichiometrical quantity necessary to oxidize all the mercaptanscontained in said hydrocarbons-to disuldcs.

2. The process of claim 1 wherein said aromatic hydrocarbon is toluene.Y

3. The process of claim l wherein said amount of air is between 1/8 and1/2 of said stoichiometrical quantity.

4. The process of claim 1 wherein said sulfuric acid is between about 93and 98% concentrated. 5. In a continuous process for sweetening souraromatic hydrocarbons of 6 to 10 carbon atoms per molecule containingmercaptans, the -steps comprising contacting them at a temperaturebetween 75 and 160 F. with between about 2 and 50 pounds per barrel ofsulfuric acid of 87-100% concentration and a controlled amount of air,mechanically agitating said mixture to producea suspension of said acidand air in said hydrocarbons, maintaining said suspension for a periodof about 1A; to 3 hours and then separating said air and said acid fromsaid suspension, said amount of air being substantially less than thestoichiometricalh quantity necessary to oxidize all the mercaptanscontained in said hydrocarbons to disulfides.

6. In a batch process for sweetening sour aromatic hydrocarbons of 6 to10 carbon atoms per molecule containing mercaptans, the steps comprisingcontacting them at a temperature between v75 F. and 160 F. with betweenabout 2 and 50 This sour toluene was admixed with 20.

pounds per barrel of sulfuric acid of 87% to 100% concentration and acontrolledamount of air, mechanically agitating said mixture to producea suspension of said acid and air in said hydrocarbons, maintaining saidsuspension for a period of about 3 to 10 hours and then separating saidair and said acid from said suspension, said amount of air beingsubstantially less than the stoichiometrical quantity necessary tooxldize all the mercaptans contained in said hydrocarbons to disuldes.

7. In a process for sweetening sour aromatic hydrocarbons of 6 to'10carbon atoms and containing mercaptans, the steps comprising contactingthem at a temperature between 75 F. and F. with between about 2 and 50pounds per barrel of relatively concentrated sulfuric acid and acontrolled amount of air, mechanically agitating the mixture to producea suspension of said acid and air in said hydrocarbons, maintaining saidsuspension for a period of about 1A; to 10 hours, separating the airfrom said suspension, and then separating the resulting spent acidk 160F. with between. about 2 and 50 pounds per barrel of relativelyconcentrated sulfuric acid and a controlled amount of air, mechanicallyagitating the mixture to produce a suspension of acid and air in saidhydrocarbons, maintaining said suspension for a period of about /a to 10hours, separating the air from said suspension, and then separating theresulting spent acid from said hydrocarbons, and distilling saidhydrocarbons to produce a pure sweetened product, said controlled amountof air being between V3 and 1/2 the stoichiometric quantity necessary tooxidize all the mercaptans contained in said hydrocarbons to disuldes.

9. 'I'he process of claim 6 wherein said aromatic hydrocarbons boilbelow about 400 F.

10. In a process for sweetening sour toluene containing mercaptans thesteps comprising contacting said toluene at a temperature between 75 and160 F. with between about 2 and 50 pounds per barrel of concentratedsulfuric acid, a controlled amount of air, and a sumcient amount o!steam to maintain the temperature of the mixture above about 85 F.,mechanically agitstinz said mixture to produce a suspension of saidacid, air, steam and hydrocarbon, maintaining said suspension for aperiod of '/3 to 10 hours. separating the air irom the suspension, andthen separating the resulting spent acid and resulting condensed steamfrom the hydrocarbons by settling, said controlled amount o! air beingbetween 5/8 and 1/2 the stoichiometric quantity necessary to oxidize al1the mercaptans contained in said hydrocarbons to disulfldes.

11. The process of claim 8 wherein said temperature is between about 110and 120 F.

GYSBERT F. nn RIDDER.

